In conventional electrophotography systems, a photoreceptor is supported by a mechanical carrier such as a drum or a belt. First, the photoreceptor is erased by exposure to an erase lamp which "bleeds" away any residual charge remaining on the photoreceptor from previous operations. The photoreceptor then is charged to a generally uniform charge, positive or negative, by subjecting the photoreceptor to a suitable charging device such as a corona or a charge roll. The charge distribution on the photoreceptor is then altered by the image-wise application of radiation, e.g., a laser, to the surface of the photoreceptor creating a latent image corresponding to the image-wise application of radiation on the photoreceptor. Toner is attracted to the photoreceptor in a pattern consistent with the charge distribution of the photoreceptor. The toner is then typically transferred, either directly or through an intermediate medium, from the photoreceptor to a receptor material or medium being printed, e.g., paper or film.
Such an electrophotography process enables the production of high quality images on the receptor material, such as film or paper. Apparatus which may utilize electrophotography include conventional laser printers, photocopiers, proofers, etc.
Monochrome printers produce a hard copy output in one toner color only, typically black. If the laser printer is to be used to print a different color, the conventional black toner cartridge is removed and replaced with a toner cartridge containing toner of another color, e.g., red. However, the laser printer still prints only a single color.
On the other hand, color printers use three primary colors, typically cyan, magenta and yellow, and in addition, optionally, black. Several techniques have been developed over the years to adapt electrophotographic techniques to use multiple colors.
U.S. Pat. No. 3,832,170, Nagamatsu et al, Method of and Apparatus for Electronic Color Photography and Photosensitive Member Used for the Same, (Canon) discloses a photosensitive member consisting basically of a supporting base, a photoconductive layer and an insulating layer dyed in a desired color for providing a color filter effect. Such photosensitive members having different color effects are provided for polychromatic reproduction on a single transferable material. Thus, the method disclosed in Nagamatsu et al requires a separate photosensitive member for each primary color plane. Not only is this method costly and bulky but significant color plane registration problems often occur due to the necessity of the transfer of the final image from multiple photosensitive members.
U.S. Pat. No. 4,578,331, Ikeda et al, Color Image Forming Method, (Ricoh) discloses an electrophotographic color image forming process wherein three light beams, each representing image information of one of three primary colors of a color document to be recorded obtained by color separation, are projected against an electrophotographic photosensitive member to form electrostatic latent images which are developed by toner of the three different colors, respectively, and printed by transfer printing, to record a color image. The image information of three colors is simultaneously written to a surface of the photosensitive member as three scanning lines either by successively writing a plurality of sets of three scanning lines each representing image information of one color or by writing image information of different colors of the same set separately in three different zones, so that the scanning lines representing image information of different colors form a repeating series of three stripes of different colors. The electrostatic latent images formed on the scanning lines are excited in positions immediately before developing sections of respective colors and developed by the toners of respective colors to produce toner images of different color which are printed by transfer printing on a transfer printing sheet. Because the method disclosed in Ikeda et al prints dry, opaque toners in separate zones, or scan lines, this system is limited in the resolution that can be provided. This loss of resolution is caused directly by the interleaving of the color planes within the page.
U.S. Pat. No. 4,728,983, Zwadlo et al, Single Beam Full Color Electrophotography, assigned to Minnesota Mining and Manufacturing Company, the assignee of the present invention, discloses a method of making high quality color prints by electrophotography. A single photoconductive drum is used together with means to erase, electrostatically charge, laser-scan expose and toner develop during a single rotation of the photoconductive drum. In successive rotations, different colored images corresponding to color separation images are assembled in register on the drum. This assembled color image is transferred to a receptor sheet in a final rotation of the drum. Because a separate pass, i.e., rotation, is required for each primary color plane, at least four passes (rotations) are needed to obtain the final four color image print. Separate passes for each of the primary color planes significantly restricts the speed which a multiple color electrophotographic printing process can achieve.
U.S. Pat. No. 4,877,698, Watson et al, Electrophotographic Process for Generating Two-Color Images Using Liquid Developer, (Xerox) discloses a process and apparatus for generating two-color images by charging an imaging member in an imaging apparatus, creating on the member a latent image comprising areas of high, intermediate, and low potential, providing an electrode having a potential within about 100 volts of that of the intermediate potential, enabling generation of an electric field and a development zone between the imaging member and the electrode, and developing the latent image by introducing into the development zone a liquid developer composition containing first toner particles of one color and second toner particles of another color, the particles being dispersed in a liquid medium, wherein the second toner particles are attracted to the high potential and the first toner particles are attracted to the low potential. The process and apparatus disclosed in Watson et al achieves a two-color image in a single pass, indeed a single developing step, but is limited to a maximum of two colors. Thus, this system would not be suitable for a standard four color image.
Thus, there is a need for a laser printing apparatus and process which will print multiple color, e.g., four color, images with improved speed, without sacrificing quality and resolution.